1. Field of the Invention
The present invention relates to a steam turbine equipped with a turbine bucket applied to a low-pressure turbine final stage and more particularly to a steam turbine used in a thermal electric power plant or the like.
2. Description of the Related Art
In recent years, steam turbines are required to deal with high-power and cost reduction. To meet such requirements, the method is often adopted to increase an area of a turbine bucket through which steam passes (hereinafter called the exhaust area) by increasing the length of the bucket of a low-pressure turbine final stage.
By increasing the exhaust area to increase the amount of steam flowing along the turbine bucket, the power of the steam turbine can be increased and power produced per casing of a low-pressure turbine can be increased. Thus, the number of low-pressure casings of the steam turbine in an output spectrum, which is conventionally two, can be reduced to one, thereby achieving a remarkable cost reduction.
One of the major problems involved in the increased length of a bucket of a low-pressure turbine final stage is that high centrifugal stress occurs in a blade portion or a dovetail during rotation of the turbine bucket. As an example that dealt with the problem, there is a case where the blade portion is made of a titanium alloy lighter than a steel-based material in order to reduce a centrifugal force acting on the blade (see JP-A-2003-65002). However, the titanium alloy is inferior to the steel-based alloy in cost or the like.